1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device that has plural oscillation sources.
2. Description of the Related Art
The lithium ion battery may be installed in various portable devices such as the digital camera. It is generally difficult to detect the remaining power of the lithium ion battery based on its voltage, and therefore, the remaining power of the lithium ion battery is normally measured by integrating the battery charge/discharge currents (e.g., see Japanese Laid-Open Patent Publication No. 2001-174534).
It is noted that a signal processing device such as a fuel gauge IC that performs signal processing operations for measuring the remaining power of a battery is being developed. Such a fuel gauge IC includes a CPU and a memory and is configured to measure the remaining power of a battery by converting detected charge/discharge current measurements into digital data and integrating the data. The fuel gauge IC uses an oscillator circuit to measure time.
Generally, an internal oscillator circuit is installed in such a fuel gauge IC in order to reduce costs and minimize the substrate mounting area. However, an oscillator circuit having an external crystal oscillating element is preferably used instead of an internal oscillator circuit in order to measure time more accurately and to thereby measure the remaining battery power more accurately. In this respect, the fuel gauge IC is preferably adapted for implementation of both the internal oscillator circuit and the crystal oscillator circuit.
Referring to FIG. 7 that illustrates oscillation signals output by the internal oscillator circuit and the crystal oscillator circuit, the internal oscillator circuit may start oscillation immediately after power is turned on at time t30 as is shown in FIG. 7 (B); on the other hand, the crystal oscillator circuit requires a time period of around 10 msec for stabilizing the oscillation waveform as is shown in FIG. 7 (C). Also, it is noted that an IC chip of a signal processing device may be reset during the time a power-on reset signal is set to high level as is shown in FIG. 7 (A).
Also, in the signal processing device as is described above, information such as trimming information stored in the memory has to be read and transferred to an analog circuit or a power supply circuit before the CPU is released from reset mode. In this respect, information is transferred from the memory to relevant circuits before reset operations of the IC chip is completed. In this case, a processing time of approximately 10 μsec may be required for transferring the information, for example.
It is noted that since the time required for stabilizing the oscillation of the internal oscillator circuit and the crystal oscillator circuit is different, the transfer timing for transferring information such as trimming information to relevant circuits and the release timing for releasing the CPU from reset mode may be different for each oscillator circuit. Therefore, activation sequences may vary depending on the oscillator circuit being used so that separate chips may have to be fabricated for the different oscillator circuits. When separate chips are fabricated to enable use of different oscillator circuits, manufacturing costs may be increased, for example.